Yeah, you can't leave Wesley out, he's the guy who took the first step towards creating the RPG.
D&D itself? Clearly in the mid-70s between the publication of the three white books and the first Basic set and the AD&D books. You've got D&D groups that are using the 3 books, maybe even copies of them, but probably not Chainmail, and likely certainaly not the other wargaming suppliments recommended for D&D. So, they're making up their own rules and creating their own structure to the game which has little to do with wargaming at all. At the end of the 70's, with Basic and AD&D in place, D&D starts becoming its own game.
If you're talking about RPGs in general, then take the above and add other early RPGs like Empire of the Petal Throne, and whatever else was being published from 1975 to 1977-1979.
Also depends on where you want to place Judge's Guild, Arduin, etc.
I come more from the Wargaming background which Gygax was a part of. Wargaming wasn't so much a part of storytelling originally, but training. From that training they came up with Kriegsspiel, a wargame to train Prussian officers. It made many interested in the facets of wargaming...
The other side of the coin was what little boys have always done, played toy soldiers. Wells merely formulated a set of rules for grown men to be able to do similar things, but on a simulation of actual wars.
Charles Roberts combined these ideas into the modern wargame and from that we can follow a direct line to the Chainmail rules, a natural evolution of the wargame idea.
When I first looked at the three booklets of D&D the first glance was more like puzzlement over the integration of the combat rules and levelling. It was nothing like what you see now...or at least very different. The roll dice against AC (or that we use a D20 to roll against AC, saves etc these days...that idea) that we used was actually a secondary option in the back.
I feel that many people really didn't know what to do with the chainmail rules that originally were the first combat system (and perhaps put in there to try to push that system (chainmail) itself instead of the D20 (using a D20:which I feel was probably MORE akin to the naval rules and the combat system Arneson had in mind). I'm not certain, as I never knew Arneson really, but from how it is described in Arneson's original system...it sounds like that would hold true...though Gygax definitely was the one to polish it up (the AC and D20 rules that Arneson had in mind instead of chainmail) and actually make it codified and understandable.
When were the actual rules where what we know as D&D today started to come to light and be codified into a definite shape which we would recognize? I think was Greyhawk.
In fact, prior to Greyhawk, I'd say Chainmail was a Wargame with some RPG elements....D&D was a Tactical wargame RPG like game, that had both wargamers and nascent Roleplayers starting it...but still in that hazy area...it was the GREYHAWK supplement that made the divide. It was Greyhawk that set the standard, that said, this is what D&D will be. This is what D&D is. This folks...this is roleplaying.
And from that the next generation of D&D, AD&D, and everything else came along...
And Greyhawk WAS Gary's campaign.
I think I'd still put Arneson as the father of Roleplaying however...Gygax definitely was the father of D&D. I can see most would also put him (Gygax) as the father of Roleplaying as well.
From what I understand, Arneson was shopping around with the system however...Gygax was just the one who happened to actually listen to him.